1. Field of the Invention
The present invention relates to an optical fiber sensor including an optical fiber in which gratings that reflect a specified wavelength of light are arrayed, a pressure sensor made up of optical fiber sensors arranged in a sheet, and an end effector on which a pressure sensor is mounted, as well as to a stress detecting method in which the optical fiber sensor, the pressure sensor and the end effector are used.
2. Description of the Related Art
Heretofore, a pressure sensor has been known in which optical fibers are arranged in a sheet that serves as a sensor, and by detecting strains in the optical fibers at a time when pressure (stress) is applied to the sheet from a body, the pressure at a location where the optical fibers are disposed is detected (see, Japanese Patent No. 3871874 and Japanese Laid-Open Patent Publication No. 2002-071323).
On the other hand, another type of pressure sensor is known that makes use of MEMS (Micro Electromechanical Systems) technology to detect as electrical signals pressures (normal stresses, horizontal stresses) that are applied in both normal and horizontal directions (see Japanese Laid-Open Patent Publication No. 2009-068988).
There are concerns that the following problems may be brought about in cases where the pressure sensors disclosed in Japanese Patent No. 3871874, Japanese Laid-Open Patent Publication No. 2002-071323, and Japanese Laid-Open Patent Publication No. 2009-068988 are applied to an end effector of a machine tool used in FA (Factory Automation) for carrying out complex assembly operations, for detecting the gripped state of an object held by the end effector, and for performing feedback controls with respect to the end effector based on pressures detected by the pressure sensors.
In the case that the pressure sensors of Japanese Patent No. 3871874 and Japanese Laid-Open Patent Publication No. 2002-071323 are applied to an end effector, while it is possible to detect the size and orientation of pressures (stresses) that are applied to the body, it is difficult to segregate and detect stresses in components having a plurality of directions. Consequently, since the state at which the body is gripped by the end effector cannot be known, the body may drop out from the end effector, and whether or not a desired assembly operation was carried out effectively cannot be confirmed.
Further, in the case that the pressure sensor of Japanese Laid-Open Patent Publication No. 2009-068988 is applied to an end effector, because the substrate that constitutes the pressure sensor is made from a silicon wafer, it is difficult for this type of pressure sensor to be attached on a curved surface region of an end effector having a curved surface. Furthermore, in case it is necessary to mold the silicon wafer in order to protect the silicon wafer from excessive stresses, as well as to protect the electric signals converted from stresses from electromagnetic noise and various surges (e.g., static electric surges caused by static electricity from human bodies or various types of machines), there are problems in that manufacturing costs rise. Still further, when one attempts to segregate the stresses applied to the body into a plurality of directional components and detect such stresses, the structure of the pressure sensor becomes complicated, and together therewith, signal processing carried out with respect to electrical signals converted from such stresses is troublesome.
Accordingly, the pressure sensor disclosed in Japanese Laid-Open Patent Publication No. 2009-068988 tends to be complicated in structure and large in scale, and of high cost, and therefore it is not easy to mount such a pressure sensor onto an end effector. Assuming that such a pressure sensor was attached to an end effector, there also is a concern that the end effector as a whole would inevitably become undesirably large in size.